1. Field of the Invention
This invention relates generally to pressure relief vents for storage tanks, and more particularly, to a relief vent for a fluid storage tank which includes an internal poppet that regulates pressure flow through the vent and can be adjusted to vary the pressure at which the vent opens to release pressure within the tank to atmosphere and prevents venting for a short period of time during sudden or rapid surge pressure conditions.
2. Description of the Related Art
Storage tanks for retaining liquids, particularly flammable or corrosive liquids, typically are provided with pressure relief vents or valves which limit the internal pressure within the tank to a predetermined value for safety. When the pressure inside the tank rises, such relief valves automatically open at a predetermined pressure value to relieve the excess pressure within the tank by venting to atmosphere. When the pressure inside of the tank is reduced below the predetermined pressure value, the relief vent or valve automatically closes and maintains pressure within the tank below the predetermined value.
During storage, use and transportation of such tanks, overturns of the tank frequently occur, such as a tank falling over or a tank rolling over in an accident. Overturns produce a very sudden liquid surge which in turn develops internal pressures within the tank which are of an extremely high value and have relatively short duration. For example, for some liquids, an overturn can create a sudden liquid surge lasting as little as two milliseconds which can develop internal pressures on the order of fifty pounds per square inch which can persist within the tank for approximately fifty milliseconds.
Surge pressures in a typical storage tank and relief vent or valve can lead to a release of large quantities of liquid from the tank by either blowing the relief vent itself or a hatch, such as a manhole cover, off the tank. Alternatively, if the relief valve assembly and other hatches hold, significant quantities of liquid may be sprayed out of the venting relief valve which can create a gaseous fog in the vicinity of the tank.
Relief valves which address such surge problems are illustrated, for example, in U.S. Pat. Nos. 4,763,688 and 5,048,553, both of said patents being owned by the assignee of the present application. Both of these patents include a snubber surge valve assembly for a relief valve which impedes the rate at which pressure can build within the relief valve upon sudden pressure rise, such as an overturn situation, to increase the time before which the relief valve can vent to atmosphere. Neither of those patents, however, disclose a vent or valve which can be adjusted before or after installation to change the pressure at which the vent or valve will release.
It therefore would be desirable to provide a relief vent for a storage tank which operates to vent to atmosphere at a predetermined pressure, momentarily prevents opening of the vent upon overturn but permits venting after a short period of time and which can be adjusted before or after assembly to the tank to vary the pressure at which the relief valve opens.